Continuous measurement of pulp properties

ABSTRACT

A method and apparatus are disclosed which permit the &#39;&#39;&#39;&#39;online&#39;&#39;&#39;&#39; measuring of the properties of a pulp, notably the specific surface, the weight-average fibre length and the consistency of a mechanical pulp. The pulp is passed through a conventional freeness analyzer, a conventional consistency meter and a specially devised stock fractionator and the readings or signals from these instruments are used, e.g. in a suitably programmed computing device, to obtain the values of the specific surface, the weight-average fibre length and the consistency of the pulp. The stock fractionator used is a self-cleaning screen of a suitable mesh size by means of which the pulp is split into a retained fraction and a through fraction. The weight ratio of one of the fractions to the feed is related to the L-factor of the pulp.

Forgacs et al.

[ 1 CONTINUOUS MEASUREMENT OF PULP PROPERTIES [76] Inventors: Otto L. Forgacs, 15 Apple Hill Rd.,

BaiedUrfe, Quebec; Alkibiadis Karnis, 487 Montcalm, Dollard Des Ormeaux, Quebec, both of Canada [22] Filed: Nov. 23, 1971 [21] Appl. No.1 201,310

[30] Foreign Application Priority Data CONSTANT HEADTANK 25 1 FEED STOCK ER T0 MlLL factor of the pulp.

[451 Apr.9, 1974 3,086,905 4/1963 Richardson....' 162/198 2,083,074 6/1937 Maass.. 162/198 3,528,281 9/1970 Cowan 162/258 X Primary E.\aminerS Leon Bashore Assistant Examiner-Richard V. Fisher Attorney, Agent, or Firm-C. A. Rowley 5 7 ABSTRACT A method and apparatus are disclosed which permit the on-line" measuring of the properties of a pulp, notably the specific surface, the weight-average fibre length and the consistency of a mechanical pulp. The pulp is passed through a conventional freeness analyzer, a conventional consistency meter and a specially devised stock fractionator and the readings or signals from these instruments are used, e.g. in a suitably programmed computing device, to obtain the values of the specific surface, the weight-average fibre length and the consistency of the pulp, The stock fractionator used is a self-cleaning screen of a suitable mesh size by means of which the pulp is split into a retained fraction and a'through fraction. The weight ratio of 1 one of the fractions to the feed is related to the L-' 6 Claims, 2 Drawing Figures DISPLAY PANEL I SPECIFIC SURFACE LT! W R L- (mm o1 weight-average R1 COMPUTING 4 m". llnglhl ::Z :f

1- T' DEVICE BU I CONSISTENCY l l l I I l l I l .J

CONTINUOUS MEASUREMENT 'OFPULP PROPERTIES 1 FIELD OF INVENTION The present invention relates to a method and apparatus for the continuous on-line monitoring of the properties of papermaking pulps. More particularly, the invention relates to a method and apparatus for determining more accurately the values of selected parameters indicative of the properties of pulps, particularly of mechanical pulps.

DESCRIPTION OF PRIOR ART ity to discriminate between the property to be mea-' sured and the other properties affecting the measurement. For example, known on-line consistency meters are affected among other things by both changes in consistency and in freeness as are the freeness meters.

It is not believed that there is any equipment available for continuously measuring'the weight average fibre length of the pulp, yet this is an important property in particular in relation to ground pulp.

SUMMARY OF INVENTION The present invention measures the properties mentioned above, namely the consistency, freeness and weight average fibre length by measuring consistency and freeness with known meters and measuring the weight average fibre length with a new deviceto obtain a value for each of these properties. By combining these three properties and solving a set of simultaneous equations determined for the specific meters used it has been found that the accuracies of the values for all three of these properties may be improved.

In some-cases, the on-line consistency meter may be omitted, i'.e., where consistency is already known or where accuracy beyondthe dependability of the online meters is.required. In the latter case, consistency may be established by a lab practice, however, continu-" ous on-line readings will then not be available, but due to the accuracy of the consistency measurement, the freeness and weight average fibre length values will also be'more accurate using the system disclosed hereinbelow. In fact, with complete accuracy of the consistency value it is believed the values for freeness and weight average fibre length will be substantially as accurate as those obtained using laboratory techniques.

The terms true or true value, when usedherein 14-, 28-, and 48-mesh screens of a Bauer-McNett fractionator, substantially as defined in the paper by O. L Forgacs, Pulp & Paper Magazine of Canada, 64C 1963, page 189. I

By mechanical pulps, in the'context of this disclosure, are understood pulps derived from wood by mechanical action in the presence of water, and the term as herein used will include groundwood, which is prepared by means of grinders, as well as refiner pulp which is generally prepared by the disc refining of -wood chips.

The above and other objects will be realized by the I present invention which provides a method for the continuous on-stream determination of the specific surface and the weight-average fibre lengthand, if desired, the true consistency of a pulp. 'Themethod comprises feeding a suspension of the pulp of known consistency to a series of measuring stations including at least a freeness analyzer and a stock fractionator susceptible of continuous operation; obtaining a reading from said freeness analyzer, said reading being anindirect measure of the specific surface of the fibres and at the same time reflecting other characteristics of said stock, including the consistency'and the. weight-average fibre length; obtaining a reading from said stock fractionator, said reading being an indirect measure of the weightave'rage fibre length and at the same time reflecting other characteristics of said stock including particularly the consistency; solving a set of simultaneous equations wherein said readings are parameters and the true'values of specific surface and weight-average fibre length are the unknowns; to obtain the said true values of specific surface and weight-average fibre length. The method also-provides for pulps of unknown consistency by further includingin said series of measuring stations a consistency meter susceptible of continuously measuring consistency; obtaining from said meter a reading of consistency, said reading being an indirect measure of the true consistency and at the same time reflecting other characteristics of said pulp including the specific surface and the weight-average fibre length; solving a set of equations wherein said reading of consistency is a further parameter and the true consistency is a further unknown; to obtain for specific-surface, weightaverage fibre length and true consistency. I

The invention also provides an apparatus for the continuous onstream determination of the specific surface and the weight-average fibre length and, if desired, the true consistency of a pulp. The apparatus comprises a freeness analyzing means and a stock fractionating means; means for feeding a pulp suspension to each of said means, said freeness analyzing means and said readings to the computing means thereby to solve the equations for the unknowns. The apparatus further comprises consistency measuring means adapted to provide a reading of consistency, said reading being an indirect measure of true consistency and at the same time reflecting other characteristics of said pulp; computing means adapted to solve a set of equations present invention, and

wherein said reading of consistency is a further parameter and the true consistency is a further unknown; and means for'fee'ding said third reading to said computing means.

The invention also provides a continuous method of measuring the weight-average fibre length, in which method a flow of a pulp suspension of known consistency and at a given rate of flow and at a given temperature is fed against a vertically mounted, vibratable, self-cleaning screen of a preselected mesh size, thereby to fractionate the pulp into a retained fraction and a through fraction, atleast one of said fractions is collected, the weight of the said collected fraction is determined, thereby to determine the weight ratio of said fraction to the pulp in said pulp suspension, said ratio being an indirect measureofsaid weight-average fibre length.

BRIEF DESCRIPTION OF DRAWINGS The invention will now be more fully understood from the following description and the accompanying drawings in which like reference numbers designate like parts or components and wherein: Y Y

' FIG. 1 is a schematic diagram of an apparatus constructed in accordance with the basic concept of the FIG. 2 is a schematic representation of one element of the present combination, namely a screening device functioning as astock fractionator or classifier.

DESCRIPTION or THE PREFERRED EMBODIMENT consistency range, preferably between 0.4 and 1.0 percent.

The dilute stock is fed through line 24 into constant head tank 25'from where it is made to flow continuonslyto the three major componen tsof the system, the

overflow from. said tank being returned to the mill sys? tem through collector line 31. Thus feed .line 14 andv valve 38 provide for the flow of' stock from the head tank to consistency metering means 11 whence the stock returns to the mill system via line 15 and colleetor line 31. Feed line 16 provides for the flow of stock to pump 32 which supplies the stock via line 17' to the -ator'via line 20 and travels back to themill system via the collector line 31. The flow to the fractionator is i measured by means of flowmeter 53, which may beany of the various flowrneters commercially available, e.g. a magnetic flowmeter; and the temperature, flow and consistency of one of the fractions, e.g. of the retained fraction, are measured respectively by means of thermometer 54, flowmeter and consistency meter 56! The temperature is measured at 51 and wherever suitable.

The consistency meter used may be one of the several types of apparatus now available in commerce and pulp suspension is delivered by a constant head device or a positive displacement pump. The flow is perpendicular to the apex of the cone-shaped free end of the spindle, and the suspension overflowing from the measuring chamberis recirculated by means of the pump.

The apparatus is generally calibrated so that a curve of consistency corresponds to the scale of torque reading which the apparatus registers. The readings will, of course, depend on process conditions such as'the temperature and the rate of flow and these conditions will therefore be standardized for the test or be allowed for 'in the reading. Any other on-line" consistency metering device may be used. All such consistency metering devices as are now available have this in common, that they measure directly some measurable property of the fibre suspension, such as e.g. resistance to flow, shear force, dielectric constant, optical properties or the like, and the reading of that particular property is translated into a consistency reading by appropriate calibration.

It will be appreciated that the readings provided by such instruments, while providing a useful indirect measure of consistency, reflect not only consistency butalso other properties of the pulp, primarily fibre length and specific surface, and therefore constitute a sort of combined index even though primarily responsive to changes in consistency. It is necessary to keep this in mind, for the operator may often be led into error by too director literal an interpretation of the readings of this type of apparatus. Thus, consistency readings provided by such apparatus will change radically when the other properties of the pulp have changed, e.g. when owing to changes in the conditions of. the process the average fibre length and/or specific surface of the fibre has changed, even though the true consistency has not changed. The reaction of the operator toa change in the consistencyreading might (and usually will) be an attempt to revert to the initial'consistency by adding or subtracting fibre to or from thesuspension. This will obviously alter the true consistency and the operator may end up with a basis weight completely different from the one desired.

Continuous freeness analyzers for on-line measuring of freeness are also available. These instruments provide'essentially an indication of the rate at which the pulp'suspension is drained or de-watered; the. physical property of the pulp primarily influencing the drainage rate is the specific surface of the fibres. An example of such an on-line freeness analyzer is the Baily freeness analyzer, based on the behaviour of the stock'suspension in a miniature decker or thickener. It consists essentially of a dandy roll rotating at constant speed in a chamber in contact with a pool of a pulp suspension at controlled consistency and maintained at a constant level. As the roll rotates, a cake is formed on the screened surface of the roll as it passes in contact with the pulp suspension, and the white water extracted from the pulp in forming the cake is removed. The rate of formation of the cake, and hence the flow of stock entering the pool to maintain it at constant level, de-

pends on the freeness of the stock. It is this flow of stock, entering the instrument in replacement of the fibres deposited on the roll, that provides the reading which serves as an index of freeness. It will be apparent thqt this flow and the reading reflecting it, will depend also on other properties or conditions, particularly on the fibre length distribution, as well as the consistency of the stock, and the temperature. Other freeness analyzers are similarly based on the measuring of the drainage or dewatering rate of the pulp suspension.

A known laboratory method of measuring the weight-average fibre length of a mechanical pulp is by the use of a Bauer-McNett fractionator. On the basis of the fibre-length classification data obtained in this apparatus, such as the percent of pulp retained on screens of a certain mesh size, namely 28 mesh, between 28 and 48 mesh, and between 48 and 100 mesh, approximate integral fibre length distribution curves of any mechanical pulp can be obtained. (0. L. Forgacs, The Characterization of Mechanical Pulps, PPRIC, 1963, Appendix B.) It was found that the fibre length distribution curve associated with any given weight-average fibre length is reasonably constant for pulps made by any given process of making mechanical pulp. This meant that fibre length could be described to a good approximation by a single number and on this basis a simplified, method of determining the weight-average fibre length was devised. In this method only the total weight of the pulp retained on the 48 mesh screen (the s0 called length factor or L) is determined, this being sufficient for a determination with sufficient accuracy of the weight-average fibre length of the pulp. A practical method of measuring continuously the L factor has now been devised for use in the present invention, in which method a flow of pulp suspension of known finite consistency (preferably of the order of 0.4 to l .5 percent) is directed against a vertically mounted screen of a suitable mesh size, thereby dividing the pulp into two fractions, a long-fibre fraction retained on the screen and a short-fibre fraction passing through the screen. The retained fraction is collected and its weight determined. Under constant conditions, i.e., for a given rate-of flow, temperature, etc., the weight ratio of fibres retained on the screen to fibres in the feed (or, for that matter, the ratio by weight of fibres passed through the screen to fibres in the feed) is a function of the weightaverage fibre length; the said ratio thus can be used to find the corresponding L factor, or, if desired, in lieu of the L-factor as an index of the said weight-average fibre length. However, it will be apparent that the amount of retained fibres, and hence the ratio of retained fibres to feed, is also affected by such other properties of pulp and process conditions, as the consistency of the feed and the flow rate. v

The device is illustrated in the drawings in FIG. 2 where numeral 60 represents a self-cleaning screen made, e.g. of nylon and mounted in a box at a suitable angle for self-cleaning, preferably vertically. The pulp suspension is fed through. line 61, the flow in which can be regulated by means of valve 62 and measured .by flowmeter 64, and the stock is sprayed onto the screen by means of nozzle 63 or a-similar device. An example of a screen assembly found suitable for this operation is a Celleco screen assembly equipped with a screen of fractions. A screen of 50 mesh size provides, in the case of mechanical pulps from Eastern Canadian and Scandinavian soft-wood species, an adequate split of the fibres for accurate measurement, but other mesh sizes can be used depending on the estimated fibre length distribution provided that each fraction represents a substantial proportion of the initial stock. The screen is generally subjected to rapid vibrations and is selfcleaning so that no mat or cake of retained fibres is formed on it.

The retained fraction (or, if preferred, the through fraction) is collected (and, if necessary, diluted byaddition of dilution water) and conducted via line 65, fitted with a temperature measuring device 66 and a flowmeter 67, into a consistency measuring device 68, of a type as described hereinabove. The weight of the pulp in the retained fraction is determined from these flow and consistency measurements. When the consistency and flow of. the feed stock is also known the weight ratio of the retained fraction (or, if preferred, the through fraction) to the feed, is easily determined. From this ratio the L-factor is determined, e.g. from a family of prepared calibration curves or in like manner, and if desired the weight-average fibre length is further determined.

Continuous readings are thus obtained for what may be called apparent consistency, freeness and L-factor (or weight-average fibre length). At the same time readings are obtained of the rate of flow of the feed to the fractionator and of the flow and consistency of the retained fraction as well as of the temperature at the various points in the system. The readings-thus obtained will provide parameter data for a set of simultaneous equations in which the true values of the pulp variables, namely the specific surface of the fibres, the weight-average fibre length and, as in the embodiment illustrated, the consistency, are the unknowns and in which the number of unknowns is equal to the number of equations, so that the set of equations is soluble. The number of parameters and of unknowns can obviously be greater than here described, this however will render the system more complicated.

A simple computing device is provided, as shown in FIG. 1, which will be adapted to solve a set of equations as described. To this device will be fed the readings obtained from the particular instruments, namely:' the temperature of the feed stock T the reading of the feedstock consistency meter R the reading of the freeness analyzer R the flow of the stock F and the temperature T flow .F,, and consistency reading R of the retained fraction as indicated by the respective broken lines 41, 42,43, 44, 45, 46 and 47. The equations relating the readings of the various instruments to the I pulp variables and the variables between themselves are set out below in equations 1 to 9'.

DEFINITION OF SYMBOLS R Feed consistency meter reading R Freeness analyzer reading R Consistency meter reading of retained fraction R Amount of pulp retained by the screen (percent 'of feed) C, True consistency of the stock (percent o.d.

fibres per unit weight of stock) C, True consistency of the retained-fraction THE EQUATIONS 3 3 r, LT! r r r) R4 100 X C Fr/C! 1 r r i,

If the process characteristics, namely stock temperatures T; and T, and flows F; and F are controlled to maintain them constant, equations (1), (2) and (9) may be inverted to provide three equations expressing the properties of consistency, specific surface and I..- factor as a linear function of the respective meter readings- R,, R and R thus:

where a, ,8, and 'y are constants which have been determined experimentally and are essentially characteristics of the apparatuses used for obtaining the meter readings It is this set of equations'that is solved by the computing device, the imputs into the device being the readings R R and R and the output from the device providing the true values of consistency, weight-' average fibre length; and specific surface of the fibres. Thesevalues can be made to appear on a display panel for easy reading by the operator.

Computing devices suitable for solving equationsof the type shown above are well known in the art. These computing devices may be of the digital type or the analog type, both of which are readily available and have tain:

.R R (1",, 0,, L (3,, F F,) v

The set of equations to be solved is:

' R, =R 0,, L,, (1,, F,, L)

I R R (C1, '1, fi.. n f, r)

Combining Equations (3),-(4), (-5), (6) and (7) we obalready found many and varied applications for process control in chemical and allied industries. The principles of an analog computer suitable for use with the present invention are described, e.g. in the textbook Process Systems Analysis and Control, by D. R. Coughanowr and L. B. Koppel (McGraw-Hill, 1965). It will be understood that in a computing device of this type the setting of the potentiometers (the function of whichis to carry out the multiplication operation) will be determined by the value of the constants a, [3 and 'y, the amplifiers will be set up to carry out the summation of the terms and so on, in a manner that will be known to the man skilled in the art.

EXAMPLE A number of different pulps (stone and refiner groundwoods as well as chemi-mechanical pulps) were run through a system consisting of a Brookfield consistency meter, a Baily freeness analyzer, and a stock fractionator consisting of a Celleco screen having a mesh size 50, substantially as described he'reinabove.

The range of variables investigated was: i

L-factor from CS Freeness do. 20 to 400 Specific surface do. 3 mlg to 9 mlg Consistency do. 3% o.d. to 1% o.d. Stock temperature do "F to l50F Using regression analysis, and for a constant value of the flows, the following set of equations was-obtained:

11,: 28.7 17,950 c, 0.15 T,' 2.88 o'; 40. L,

R 47.1 (0.074/C;) '4- 0.112 T,+ 8.8 07+ 269 L,-

36.8 a ls,

R,= 20.5 0.15 r, 1.73 o' 41 L, 2,900 c,+ 337 X104 2.7 x (3,13, 162.7 x10 0, 3.76

x 10' no,

The Canadian Standard Freeness was calculated from Equation (8) which can be written in the form G,= 218 66.6 07-i- 710 L,- 88 L,o, 5,7 07

The stock temperature was controlled to T; 140 F and T, 100 F, by regulating the dilution water temperature. Under these conditions, inversion of Equations (1a), (2a) and (9a) yields Equations (10a), 1 la) and (12a) in which the properties C], 0', and L} are expressed as a function of the meter readings R R and R These equations in linear form are:

C] 0.55 0.0074 R, 0.0042 R 4- 0.010 R,

0,= 1.7 0.39 R, 0.014 R 0.33 R,

L,= 1.0-0.15 R, 0.23 R2+O.44 R

' An unknown mechanical pulpjwas then passed through the system and readings R R and R registered by the instruments. The readings were:

R2 R, i 67 The recorded values for R R and R were substituted in equations (10a), (10a) and. (12a), and on solving.

The advantages of the present invention will be easily perceived by those skilled in the art. The system allows the true values of the important stock variables to be obtained continuously, quickly and substantially automatically. Moreover, the measurements are carried out on bulk samples of pulp (not on individual fibres), en-

suring that a representative sample of the main stock is being taken. The system incorporates a continuous method of measuring weight-average fibre length, as opposed to number-average fibre length, the former being far more important for the characterization of mechanical pulps.

What we claim is:

l. An apparatus for continuous on-stream determination of the specific surface and the weight-average fibre length of a pulp which comprises a freeness analyzing means and a stock fractionating means arranged in parallel, means for feeding a sample of said pulp in suspension to each of said means, said freeness analyzing means providing a fir'stsignal, said first signal being a measure of the specific surface of the pulp and at the same time reflecting the weight average fibre length of said pulp, said stock fractionating means .providing a second signal, said second signal being a measure of the weight-average fibre-length of the pulp and at the same time reflecting the specific surface of said pulp, computormeans, said computor means computing from said signals true values of said specific surface and said weight-average fibre length and means for feeding said first and said second signals to said means for computing thereby to obtain'said true values of specific surface and weight-average fibre length of the pulp.

2. An apparatus as-defined in claim 1 further comprising consistency measuring means. providing a third signal, means for feeding a sample of said pulp in suspension to said consistency measuring means, said third signal being a measure of consistency and at the same time reflecting said specific surface and said weightaverage fibre length of said pulp, said consistency measuring means being arranged in parallel with the free ness analyzing means and the stock fractionating means, means for feeding said third signal to said com putor means, said computor means computing from said first, second and third signals, true values of said specific surface, said weight-average fibre length and of said consistency.

3. An apparatus as claimed in claim 2 wherein said 5. An apparatus as claimed in claim 2 wherein said stock fractionating means is a self-cleaning vibrating vertical screen of a mesh size to divide said pulp suspension fed to saidstock fractionating means into substantially equal fractions.

6. An apparatus as defined in claim 5 wherein said stock fractionating means further comprises means measuring the consistency and flow of one of said fractions. 7 I 

1. An apparatus for continuous on-stream determination of the specific surface and the weight-average fibre length of a pulp which comprises a freeness analyzing means and a stock fractionating means arranged in parallel, means for feeding a sample of said pulp in suspension to each of said means, said freeness analyzing means providing a first signal, said first signal being a measure of the specific surface of the pulp and at the same time reflecting the weight average fibre length of said pulp, said stock fractionating means providing a second signal, said second signal being a measure of the weight-average fibre length of the pulp and at the same time reflecting the specific surface of said pulp, computor means, said computor means computing from said signals true values of said specific surface and said weight-average fibre length and means for feeding said first and said second signals to said means for computing thereby to obtain said true values of specific surface and weight-average fibre length of the pulp.
 2. An apparatus as defined in claim 1 further comprising consistency measuring means providing a third signal, means for feeding a sample of said pulp in suspension to said consistency measuring means, said third signal being a measure of consistency and at the same time reflecting said specific surface and said weight-average fibre length of said pulp, said consistency measuriNg means being arranged in parallel with the freeness analyzing means and the stock fractionating means, means for feeding said third signal to said computor means, said computor means computing from said first, second and third signals, true values of said specific surface, said weight-average fibre length and of said consistency.
 3. An apparatus as claimed in claim 2 wherein said freeness analyzing means is a continuous freeness analyzer measuring the drainage rate of said pulp suspension.
 4. An apparatus as claimed in claim 2 wherein said consistency measuring means is a continuous consistency meter measuring resistance to the flow of the pulp suspension.
 5. An apparatus as claimed in claim 2 wherein said stock fractionating means is a self-cleaning vibrating vertical screen of a mesh size to divide said pulp suspension fed to said stock fractionating means into substantially equal fractions.
 6. An apparatus as defined in claim 5 wherein said stock fractionating means further comprises means measuring the consistency and flow of one of said fractions. 